R. V. Gulyaev

Low-temperature oxidation of carbon monoxide on Pd(Pt)/CeO2 catalysts prepared from complex salts

Catalysts containing cerium oxide as a support and platinum and palladium as active components for the low-temperature oxidation of carbon monoxide were studied. The catalysts were synthesized in accordance with original procedures with the use of palladium and platinum complex salts. Regardless of preparation procedure, the samples prepared with the use of only platinum precursors did not exhibit activity at a low temperature because only metal and oxide (PtO, PtO2) nanoparticles were formed on the surface of CeO2. Unlike platinum, palladium can be dispersed on the surface of CeO2 to a maximum extent up to an almost an ionic (atomic) state, and it forms mixed surface phases with cerium oxide. In a mixed palladium-platinum catalyst, the ability of platinum to undergo dispersion under the action of palladium also increased; as a result, a combined surface phase with the formula PdxPtyCeO2 − δ, which exhibits catalytic activity at low temperatures, was formed.

Effect of preparation procedure on the properties of CeO2

The effect of preparation procedure on the physicochemical and catalytic properties of CeO2 was studied. Differences in the electronic and structural characteristics of CeO2 depending on preparation procedure and treatment temperature were found using X-ray diffraction analysis, transmission electron microscopy, UV-visible electronic spectroscopy, and X-ray photoelectron spectroscopy. With the use of the temperature-programmed reaction with CO, it was demonstrated that CeO2 samples with a high concentration of point defects—oxygen vacancies caused by the presence of Ce3+—were characterized by an increased mobility of bulk oxygen. The samples of CeO2 with a high concentration of structural defects—micropores of size 1–2 nm and stepwise vicinal faces in crystallites—exhibited a high catalytic activity in the reaction of CO oxidation.

Synthesis and physicochemical characterization of palladium-cerium oxide catalysts for the low-temperature oxidation of carbon monoxide

The effect of CeO2 preparation procedure on the electronic and structural states of the active component of Pd/CeO2 catalysts and their activity in the low-temperature reaction of CO oxidation was studied. The following two nonequivalent states of palladium were detected in the catalysts having low-temperature activity using XPS and IR spectroscopy: Pd0(Pdδ+) as the constituent of a palladium-reduced interaction phase and Pd2+ as the constituent of a palladium-oxidized interaction phase PdxCeO2 −δ. It was found that the procedure used for preparing a CeO2 support considerably affected the formation of these phases and quantitative ratios between them. It was demonstrated that the palladium-oxidized interaction phase was responsible for low-temperature activity, whereas the palladium-reduced interaction phase was responsible for activity in the region of medium and high temperatures.

Low-temperature oxidation of carbon monoxide over (Mn1 − xMx)O2 (M = Co, Pd) catalysts

Abstract(Mn1 − xMx)O2 (M = Co, Pd) materials synthesized under hydrothermal conditions and dried at 80°C have been characterized by X-ray diffraction, diffuse reflectance spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, and adsorption and have been tested in CO oxidation under CO + O2 TPR conditions and under isothermal conditions at room temperature in the absence and presence of water vapor. The synthesized materials have the tunnel structure of cryptomelane irrespective of the promoter nature and content. Their specific surface area is 110–120 m2/g. MnO2 is morphologically uniform, and the introduction of cobalt or palladium into this oxide disrupts its uniformity and causes the formation of more or less crystallized aggregates varying in size. The (Mn,Pd)O2 composition contains Pd metal, which is in contact with the MnO2-based oxide phase. The average size of the palladium particles is no larger than 12 nm. The initial activity of the materials in CO oxidation, which was estimated in terms of the 10% CO conversion temperature, increases in the following order: MnO2 (100°C) < (Mn,Co)O2 (98°C) < (Mn,Co,Pd)O2 (23°C) < (Mn,Pd)O2 (−12°C). The high activity of (Mn,Pd)O2 is due to its surface containing palladium in two states, namely, oxidized palladium (interaction phase) palladium metal (clusters). The latter are mainly dispersed in the MnO2 matrix. This catalyst is effective in CO oxidation even at room temperature when there is no water vapor in the reaction mixture, but it is inactive in the presence of water vapor. Water vapor causes partial reduction of Mn4+ ions and an increase in the proportion of palladium metal clusters.

Synergetic effect in PdAu/CeO2 catalysts for the low-temperature oxidation of CO

Gold-palladium catalysts supported on cerium oxide were synthesized with the double complex salts. X-ray photoelectron spectroscopy (XPS) and other physicochemical methods (TEM, TPR) were used to demonstrate that synthesis of highly active palladium catalysts requires the oxidative treatment stimulating the formation of a catalytically active surface solid solution PdxCe1−xO2, which is responsible for the lowtemperature activity (LTA) in the reaction CO + O2. In the case of gold catalysts, active sites for the lowtemperature oxidation of CO are represented by gold nanoparticles and its cationic interface species. Simultaneous deposition of two metals increases the catalyst LTA due to interaction of both gold and palladium with the support surface to form a Pd1−xCexO2 solid solution and cationic interface species of palladium and gold on the boundary of Pd-Au alloy particles anchored on the solid solution surface.

X-ray photoelectron investigation of charge distribution in copper(II) phthalocyanine complexes

The charged states of atoms in unsubstituted copper(II) phthalocyanine (CuPcH16) and hexade-cafluorinated copper(II) phthalocyanine (CuPcF16) complexes and in thin films of them deposited on silicon substrates by vacuum thermal evaporation are investigated by X-ray photoelectron spectroscopy (XPS). The C(1s), N(1s), Cu(2p) core level energies and the charged states of atoms in the studied complexes are calculated using the DFT method. The performed experimental study and theoretical calculations show that the introduction of electron acceptor substituents into benzene rings mostly affects the atoms of benzene rings and insignificantly affects the charge state of nitrogen atoms in the pyrrole ring.

X-ray spectral and photoelectron study of the electronic structure of copper phthalocyanine and its fluoro-substituted analog

A complex experimental study of Cu(2p3/2), Cu(2p1/2) photoelectron and Kα1,2 and Lα1,2 X-ray emission spectra of copper in copper phthalocyanine CuPcH16 and its fluoro-substituted analog CuPcF16 is carried out. A charge transfer model is used to interpret the spectra. It is shown that Kα1 and Kα2 lines of the spindoublet of copper have a complex structure due to the processes of metal-to-ligand charge transfer. The role of a satellite in the formation of emission lines is revealed.

Films of the PdxCe1−xO2 solid solution as a model object for the XPS study of the surface chemistry of Pd/CeO2 catalysts

The paper demonstrates a new technique for preparing nanostructured films of the PdxCe1−xO2−x−δ solid solution. The obtained films are similar to real powder catalysts by their chemical and structural properties and are conductive, which is favorable for the XPS methoda. The effect of the CO+O2 reaction on the oxidized and reduced surfaces is studied using these model objects. The presence of the reversible “PdxCe1−xO2−x−δ ↔ surface palladium forms” transition occurring already at room temperatures in the CO+O2 reaction is shown. Deconvolution of the O1s spectra made it possible for the first time to separate experimentally the contributions from OH− hydroxyl and CO32− carbonate surface groups.

Role of the support in the formation of the properties of a Pd/Al2O3 catalyst for the low-temperature oxidation of carbon monoxide

The Pd/Al2O3 catalysts were prepared by the impregnation of aluminum hydroxide, which was synthesized by precipitation in the presence of polyvinyl alcohol, with a solution of palladium nitrate and were heat-treated at different temperatures. The resulting samples were characterized by X-ray diffraction, electron microscopy, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy and were tested in CO oxidation in two modes: in a temperature-programmed reaction and under isothermal conditions at 20°C in the absence and in the presence of water vapor. The activity of the catalysts in the former mode was almost independent of support preparation conditions, but it was different in the latter mode. The catalyst whose support was obtained in the presence of polyvinyl alcohol and treated at 300°C in an atmosphere of nitrogen exhibited the highest activity in CO oxidation at 20°C. In the absence of water vapor from the reaction mixture, the initial conversion of CO reached 40% and then decreased. In the presence of water vapor, a continuous increase in the conversion of CO to 88% was observed, and the activity was stabilized at this level. The smallest size of palladium metal nanoparticles, the nearly monolayer carbon surface coverage, and the presence of OH groups, which are formed upon the dissociation of water present in the reaction mixture, facilitate an increase in activity.

Simulation of X-ray emission and photoelectron spectra of H2Pc using the density functional method

X-ray emission and photoelectron spectroscopy and quantum chemical calculations are applied to study the electronic structure of H2Pc. A comparative analysis of the experimental and theoretical data of the energy spectrum and the partial composition of HOMO for phthalocyanine is performed. It is shown that the H2Pc HOMO is mainly composed of carbon 2pπ AO — Cγδ. The best agreement between the experimental partial density distribution in HOMO and the theoretical one is observed for the calculation by the density functional method in the Z+1 approximation. The DFT-ZORA method with LB94 model functional and the QZ4P basis set enables a high-accuracy calculation of the energies of 1s levels of non-equivalent atoms in H2Pc relative to each other.